The present invention is directed to a new type of solid-state breaker that uses magnetic coupling instead of a crossed connection to accomplish an automatic response to an electrical fault.
Since the great debate between Thomas Edison and Nikola Tesla our nations power system has operated on alternating current (AC). This was chosen over direct current (DC) because of the need to step up the voltage to a high value using transformers for long-distance power transmission. Traditional transformers available at the time only operated on AC. Nowadays, many energy sources such as solar panels, fuel cells, batteries, etc. supply a DC voltage. Also, DC/DC power converters are common-place for transforming the voltage and interfacing these DC sources to a larger system. Because of this, local DC power systems, or micro grids, have become increasingly more popular. Furthermore, interfacing a wind power generator to a DC system is much simpler than interfacing to an AC system since an AC/DC conversion is needed for the former and an AC/DC/AC conversion is needed for the latter. Although the energy sources and power conversion is readily available for DC power systems, circuit breakers are not. Breaking an AC circuit is much easier than breaking a DC circuit. The DC circuit has a constant current without a zero crossing, and thus breaking this circuit causes arcing that cannot be extinguished. Options for making a DC circuit breaker include a hybrid mechanical/electrical design, a solid state breaker, and z-source breakers as described herein.
C. Meyer, M. Kowal, and R. W. De Doncker, “Circuit Breaker Concepts for Future High-Power DC-Applications,” IEEE Industry Applications Society Conference, volume 2, pages 860-866, 2005 describes a hybrid mechanical/electrical breaker. A mechanical switch is placed in parallel with solid-state devices. During normal operation, the mechanical switch conducts the main current. A fault detection circuit opens the mechanical switch and diverts the current through a power transistor, then to a metal-oxide varistor. This device clamps the breaker voltage until the fault current can be reduced by the system inductance. The primary advantage of the hybrid breaker is low power losses during normal operation. A disadvantage of this circuit is the time needed for fault detection and switch turn-off leading to the source therefore a significant amount of current can be experienced before the fault can be isolated.
A. Pokryvailo and I. Ziv, “A Hybrid Repetitive Opening Switch For Inductive Storage Systems And Protection Of Dc Circuits,” IEEE Power Modulator Symposium, High-Voltage Workshop, pages 612-615, 2002 demonstrates a solid-state breaker. A silicon-controlled rectifier (SCR) conducts the current. A resonant L-C circuit is also part of the breaker and has the capacitor pre-charged. Upon detection of a fault, a secondary switch places the L-C circuit in parallel with the SCR. This causes a resonance whereby the SCR current goes to zero and the SCR switches off; isolating the faulty load. After opening, a third inductive branch is connected to the L-C circuit causing a reversal of polarity in the capacitor so that the breaker is reset and ready for operation again. The solid-state breaker offers fast switching times which is important in DC power systems so that the source current does not build up to excessive levels. The primary disadvantage of this circuit is the on-state power losses.
R. Schmerda, R. Cuzner, R. Clark, D. Nowak, and S. Bunzel, “Shipboard Solid-State Protection: Overview and Applications,” IEEE Electrification Magazine, volume 1, issue 1, pages 32-39, 2013 presents a special type of solid-state circuit breaker. After fault detection, a main path transistor disconnects the faulty load. This circuit has an additional diode path which picks up the load current after the source is disconnected. The concept has been extensively developed and tested for Naval shipboard power systems. This circuit has an advantage of being extremely fast. Normal on-state power losses are an issue as is the need for fault detection.
K. A. Corzine and R. W. Ashton, “A New Z-Source Dc Circuit Breaker,” IEEE Transactions on Power Electronics, volume 27, number 6, pages 2796-2804, June 2012 introduced a new concept in the solid-state breaker circuits called the z-source breaker. An added crossed connection of inductors and capacitors causes the breaker to automatically switch off in response to a transient change in the load current. This circuit has the advantage of quick fault isolation that is typical of solid-state breakers. This circuit has the additional advantage that detection of the fault is not necessary. Furthermore, the source does not experience a fault current and instead, the source current is pushed to zero during a fault. The primary disadvantage of this circuit is the on-state power losses.
A. H. Chang, A. Avestruz, S. B. Leeb, and J. L. Kirtley, “Design of DC System Protection,” IEEE Electric Ship Technologies Symposium, pages 500-508, April 2013. Shortly after the introduction of the z-source breaker, this group of MIT researchers presented a variation. The primary advantage of this circuit is that it has a common-ground connection between the source and load and has the desirable frequency response property of a low-pass filter.
FIG. 1A shows a typical arrangement of a circuit breaker inserted between a source and load. In this circuit, the source current is monitored for fault current detection. Alternatively, a capacitor can be connected to ground within the breaker as shown in FIG. 1B. This method is good for detecting transient currents and is used in motor drives for detection of shoot-through. That is, a small capacitor in series with some type of current sensor can be connected to the DC bus of a drive. Shoot-through faults create an impulse of current in this capacitor and the detection can immediately switch off the drive's gate signals. Likewise, a short path could be added to any type of DC circuit breaker for fast detection of faults. Instead of monitoring the main path current (between source and load) and allowing the source to experience the fault current for a while, the short path between the added capacitor and load readily indicates the fault.
In spite of the ongoing effort those of skill in the art still do not have a suitable option for a circuit breaker suitable for DC applications. An improved DC circuit breaker is provided herein.